When we decided to build our Connor Homes kit house, we had visions of a super-efficient, modern home wrapped in the latest insulation technology. We’d read all about spray foam, tight building envelopes, and energy efficiency. We were going to do this right.
We sort of did. Maybe. I’m still not entirely sure.
The Plan: Go Modern, Go Tight
Coming from NYC apartments where you could feel the draft through the windows in winter and hear every conversation in the apartment next door, we wanted our new house to be different. Quiet. Comfortable. Efficient. So we went with spray foam insulation and committed to a tight building envelope.
The building code required pretty high R-values for our house because we had so many windows. And by “so many,” I mean depending on how you count (do you count the triple window in the kitchen as one window or three?), we had 27-29 windows, plus French doors and two entry doors. Our house was basically a glass box that needed to compensate with serious insulation everywhere else.
We chose soy-based foam, which sounds environmentally friendly until you realize it’s still polyurethane with a tiny bit of soy oil in it. The marketing made a big deal about it being “plant-based” and “renewable.” They also claimed rodents don’t like it, which seemed important given that we were building in a field surrounded by woods. Whether the soy actually repels rodents or attracts them is one of those things I never quite got a straight answer on.
The Hot Attic vs. Cold Attic Dilemma
Here’s where things got confusing fast. We needed R-38 in the attic, and we had to decide whether to do a “hot attic” or “cold attic” — industry terms that nobody bothered to explain in plain English until we were already knee-deep in the decision.
A cold attic is the traditional approach: insulate the attic floor, vent the attic space, treat the attic as outside your building envelope. A hot attic is the newer approach: spray foam directly to the underside of the roof deck, seal everything up, bring the attic inside your conditioned space.
The internet was full of conflicting information. Some sources said you needed closed-cell foam for hot attics because it works as a vapor barrier. Others said open-cell was fine if you vented between the rafters. Still others said venting wasn’t necessary at all with spray foam. It was like trying to solve a puzzle where everyone insisted they had the answer but nobody’s answer matched.
Our contractor added to the confusion. He told us he didn’t recommend closed-cell foam on the rafters because the rafters would shrink over time and the rigid closed-cell wouldn’t move with them, leaving gaps. Instead, he suggested a layer of closed-cell first, then filling the rest with open-cell.
Looking back at my notes now, this doesn’t make much sense to me. If the rafters are going to shrink and create gaps with closed-cell foam, why would putting a layer of it down first help? Wouldn’t you still get gaps at that layer? And if moisture could penetrate those gaps, wouldn’t that defeat the whole vapor barrier purpose of the closed-cell layer?
But what did we know? We were two people from Manhattan trying to build a house while working in the city all week.
The Hybrid Solution (We Think)
We ended up going with what the contractor called a “heavier” open-cell foam that would work as a vapor barrier. I’m assuming we did the right thing, but fifteen years later, I’m still not 100% confident.
Here’s what actually got installed, thanks to the invoice I saved:
- Attic rafters: Agribalance open-cell foam, R-38 at 8.5 inches, peak to plate
- Attic gable walls: same Agribalance open-cell
- Exterior walls, both floors: Sealection 500 open-cell, R-21 at 5.5 inches
- Band joists: Sealection 500 open-cell, R-21 at 5.5 inches
- Basement: Knauf Ecco batts, R-19, formaldehyde-free (batts were fine here — we weren’t trying to create an air seal in the basement)
- Floor/ceiling between levels: R-13
The crew from Sealrite Spray Foam did good work. They protected the windows and mechanical systems with plastic sheeting, foamed around all the window and door jambs with low-expanding window foam, caulked the plates and jack studs, and installed cardboard baffles at the eaves. They also left the window covering up when they finished, which meant we didn’t have to mask the windows again when we sprayed paint later. Small win.
The Radiant Floor Surprise
Here’s something we didn’t anticipate. Because we were doing radiant floor heating, we had to insulate between the floors and ceilings to make the system work efficiently.
This added unexpected cost, but worse, it prevented the heat from spreading naturally through the house. We’d imagined the radiant heat warming the basement and attic a bit, making the whole house more comfortable. Instead, the insulation trapped heat on each floor — more efficient, but each floor became its own thermal island. The basement stayed cold. The attic stayed cold. Nothing spread.
Would they have recommended floor/ceiling insulation anyway if we’d gone with European radiators instead? Probably. But it wasn’t what we wanted, and it wasn’t what we’d budgeted for.
What We Wish We’d Had
Looking back, I would have preferred cellulose. It’s less expensive. It’s actually made from recycled materials, not just “plant-based” with quotation marks. And it doesn’t carry the flammability concerns I still worry about with foam.
But you can’t spray cellulose on rafters. You can only blow it onto attic floors. Since we were doing a hot attic with the insulation at the roof deck, we had no choice but to go with spray foam.
I also wish the ZIP System had been available in 2009. The Connor Homes kit came with traditional house wrap, which was fine for the time and probably worked well with the spray foam. But ZIP — structural sheathing with the weather barrier built in — would have been one fewer thing to worry about.
The real weak points of the envelope, though, are the windows and doors. The spray foam did its job. The walls are tight. But we cheaped out on Pella windows (see Part 11 for that disaster) and went with Simpson wooden doors for the “old house vibe.” All the air leakage in this house happens at the windows and doors.
Proof: when there’s snow piled against the windowsills blocking the infiltration, our house is extra toasty and there’s no condensation on the glass. The rest of the time? Moisture city.
Damn you, Pella.
The Great Winter Moisture Disaster
Once we had the spray foam installed, the house became incredibly tight. Which is exactly what we wanted.
It was also a disaster.
The spray foam went in during late winter 2009/early 2010, and the house was immediately full of moisture. This wasn’t a leak. This was moisture from the foam curing, from the wood framing drying out, from everything in a new house releasing water vapor into the air.
In a normal house with air leaks, this moisture escapes naturally. In our super-tight, spray-foamed house, it had nowhere to go. We couldn’t just open the windows to vent it because the pipes would freeze. So the moisture just stayed. And built up. And made everything damp.
This is why you need mechanical ventilation in a tight house. Which we planned for, with an HRV. But remember from Part 10? The contractor undersized the HRV. So even running it 24/7 wasn’t enough to catch up with the moisture.
The lesson is timing. Frame in spring so the wood has the summer to dry out. Insulate in fall. Keep windows open during drywall and paint to vent that additional moisture before you close up the envelope for good.
We did not do this. We did the opposite of this.
Did the Air Sealing Actually Work
Yes. The spray foam created an air barrier in the walls and roof. Fifteen years later, the foam is still in place, not sagging, not deteriorating. The only places I’ve ever noticed air leaking are — surprise — the windows and doors.
So the envelope, as built, works exactly as intended. The problem is what we put in the holes we cut into it.
Environmental and Safety Concerns
I said earlier that we chose soy-based foam because it sounded environmentally friendly. Fifteen years later, I have serious questions about whether spray foam — any spray foam, soy-based or not — is actually good for the environment.
It’s made from petrochemicals. It off-gasses during installation, which is why the installers wear full protective gear and you can’t be in the house during application. And while it’s recyclable in theory, in practice it’s never getting recycled — it’s permanently adhered to your house.
The “plant-based” marketing always felt like greenwashing to me, and time hasn’t made me feel any better about it.
I also worry about flammability. Spray foam is combustible. It will burn if exposed to direct flame. The installers and manufacturers will tell you it has flame retardants and meets building code fire ratings, which is true. It’s covered by drywall, which acts as a 15-minute thermal barrier. And in a real fire, lots of things in your house will burn anyway — wood framing, furniture, carpets, basically everything.
But still. It’s foam. Made from petrochemicals. In my walls. That feels different from wood studs.
Is this a rational concern? Probably not. Would I make a different choice today? I honestly don’t know.
Would We Do It Again
If I were building today, the insulation strategy would be mostly the same. Spray foam for air sealing — it really does work. Hot attic design — it brings the HVAC inside the envelope, which is more efficient. And a professional crew, because this is not a DIY project.
What I’d change: time the build so the framing dries through a full summer before insulation goes in. Get an actual HVAC engineer to size the HRV instead of trusting the contractor’s gut. Spend the spray-foam savings on triple-pane windows that actually work. Maybe pay for closed-cell on the roof deck despite the cost, just for peace of mind on moisture. And ask more questions about the vapor barrier strategy instead of nodding along to the contractor’s logic.
The spray foam itself has been fine. The problems we’ve had have been about timing, ventilation sizing, and the windows. None of which are the foam’s fault.
Grade: B. The spray foam worked. Fifteen years in, the Agribalance and Sealection 500 are still there, still doing their job, still tight. Sealrite did good work. The walls don’t leak. The problem isn’t the insulation. The problem is that we cured the foam in a New York winter with nowhere for the moisture to escape, that the HRV was undersized so it couldn’t catch up later, and that we punched twenty-seven holes in the envelope and filled them with Pella. We built the safe. We left the door open.
Next up: Part 13 — Drywall.
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